Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.
The separation of CO2 from flue gases in power stations, cement kilns and in steel manufacturing allows these industrial activities to proceed with the use of fossil fuels, while reducing the emissions of the most important greenhouse gas, i.e. CO2. Although several different processes are currently under development for the separation of CO2 from flue gases, chemical absorption processes using aqueous solutions of chemical absorbents are the leading technology, mainly because of its advanced state of development. While it is already available at low CO2-removal capacities, it is not at the scale necessary for large scale industrial operation. Scaling up the process is therefore a major challenge. The typical flow sheet of CO2 recovery using chemical absorbents is shown in FIG. 1.
After cooling, the flue gas 100 is brought into contact with the chemical absorbent in an absorber 102. A blower 103 is required to pump the gas through the absorber after passing through a cooler 104 at temperatures typically between 40 and 60° C. whereby CO2 is then bound by the chemical absorbent in the absorber 102. After passing through the absorber 102, the flue gas undergoes a water wash section 113 to balance water in the system and to remove any droplets of vapour carried over and then leaves the absorber 102. The “rich” absorbent solution, which contains the chemically bound CO2, is then pumped to the top of a stripper 107, via a heat exchanger 105. The regeneration of the chemical absorbent is carried out in the stripper 107 at elevated temperatures (100-140° C.) and pressures between 1 and 2 bar. The stripper 107 is a gas/liquid contactor in which the rich absorbent is contacted with steam produced in a reboiler 108.
Heat is supplied to the reboiler 108 to maintain the regeneration conditions. This leads to an energy penalty as a result of the heating up the solution to provide the required desorption heat for desorbing the chemically bound CO2 and for steam production which acts as a stripping gas. Steam is recovered in a condenser 109 and fed back to the stripper 107, whereas the CO2 product gas 110 leaves the condenser 109. The heat of condensation is carried away in cooling water or an air cooling device. The CO2-product 110 is a relatively pure (>99%) product, with water vapour being the main other component. Due to the selective nature of the chemical absorption process, the concentration of inert gases is low. The “lean” absorbent solution 111, containing far less CO2 is then pumped back to the absorber 102 via the lean-rich heat exchanger 105 and a cooler 112 to bring it down to the absorber temperature level. CO2 removal is typically around 90%.
The energy requirement of a chemical absorption process mainly stems from the heat supplied to the reboiler 108. This heat is used to produce steam from the lean solution 106 which acts a stripping gas, i.e. it keeps the partial pressure of CO2 sufficiently low to provide a driving force for the desorption process. The steam is also the carrier of thermal energy which, through its condensation, releases the energy required to desorb CO2 and to heat up the chemical absorbent through the desorption column 107. The amount of steam generated in the reboiler 108 should be kept as a low as possible, but some of the steam will always inevitably be lost from the desorption unit with the CO2-produced and this represents an energy loss, as the steam is usually condensed and the energy is carried away in the cooling water.
There exists a need to provide a process and apparatus that is more energy efficient than the present process. Various approaches have been suggested in the prior art.
WO 2007/075466 discloses an approach in which the lean chemical absorbent exiting from the bottom of the desorption column, is sent to a flash vessel at lower pressure than the equilibrium pressure. The lean solution has a low CO2 partial pressure; hence the vapour produced is predominantly steam. The steam can be recompressed and injected into the desorber to provide additional steam for stripping and heating of the chemical absorbent in the desorption column. This requires the addition of two pieces of equipment: flash vessel and a compressor. This adds to capital and operating expenses. Furthermore, the addition of a further compressor adds to the overall energy consumption—thus detracting from energy savings, which is undesired.
U.S. Pat. No. 4,152,217 discloses the recovery and reuse of heat by using a heat exchanger to extract heat energy from the CO2/steam mixture exiting the desorption column through heat exchange with the rich absorbent solution entering the desorption column. This allows the latent heat of condensation to be recovered as sensible heat in the rich absorbent. The teachings of this patent show however that a split of the rich absorbent flow into two streams is needed, which is complicated from a process flow and control perspective.
U.S. Pat. No. 4,444,571 discloses a method to recover latent heat from gas/steam mixtures that uses a membrane which is selectively permeable towards steam over other gases contained in the mixture. Permeate which predominantly contains steam is recompressed and injected into the bottom section of the desorption column. Although suitable membranes are available, the process requires additional energy, which is undesirable.
The state-of-the-art methods to improve the energy performance of the desorption column are all limited because they only address a single improvement step and assume the usual process lay-out as shown in FIG. 1.
It is an object of the invention to address at least some of the above aforementioned short-comings of the prior art.